That was a conscious decision by the XFCE devs. They stated they were not ready to do a technological overhaul, and update to GTK-4 (which does support Wayland)

That was a conscious decision by the XFCE devs. They stated they were not ready to do a technological overhaul, and update to GTK-4 (which does support Wayland) for the 4.16 release cycle. As they just released 4.16 last month, we should see an announcement regarding dependencies for 4.18 anytime now.